The present invention relates in general to a displacement generation device using a piezoelectric displacement element having a piezoelectric effect for generating a distortion upon a change in an applied voltage and, in particular, to a displacement generation device using a piezoelectric displacement element which vibrates in response to a control voltage signal to achieve a desired vibration mode.
A bimorph cell is generally known as one of the typical elements which has a piezoelectric effect. The bimorph cell comprises piezoelectric plates which are cemented together in a stacked manner. The bimorph cell may be constructed such that piezoelectric plates are bonded to a metal plate or the like. The piezoelectric element of this type is effectively used as a means for generating a slight periodic displacement in accordance with a change in an applied voltage, and is applied in a variety of fields. For example, the bimorph cell of this type is used as a laser beam deflection element in an optical video disk system or as a video head displacement element in a helical scan type VTR. In the optical video disk system, a mirror is mounted at the distal end of the bimorph cell. When a proper voltage signal is applied to this piezoelectric element, the cell is distorted changing the position of a mirror so as to deflect the laser beam. In the helical scan type VTR, the bimorph cell causes proper displacement of the video head, thereby performing auto-tracking.
In a special application of the bimorph cell, a deflection unit is proposed to periodically change the light-receiving position of a solid-state image sensor such as a charge-coupled device (CCD) with respect to an incoming optical image so as to obtain a high-resolution image. A deflection unit of an interline transfer type solid-state image sensor is disclosed in U.S. patent application Ser. No. 484,511 by Chiaki TANUMA et al. According to this reference, a light-receiving surface of the CCD is displaced or vibrated by a bimorph cell at a proper resonance frequency with respect to the incident image beam. The number of picture elements obtained by the CCD can be effectively increased. Therefore, without actually increasing the number of picture elements of the CCD, a high-quality image can be obtained.
However, the actual displacement of the bimorph cell changes in accordance with its ambient temperature and the type of piezoelectric material used. However, a temperature coefficient of the displacement falls within the range of 2,000 to 10,000 ppm/.degree.C. (-20.degree. C. to 60.degree. C.). For example, when the solid-state image sensor is mounted in a VTR camera, an electronic camera or the like, the temperature changes in accordance with the given circumstances. When the temperature changes, the displacement also changes, as described above. As a result, the desired displacement cannot be obtained, resulting in inconvenience. Therefore, a high-resolution image cannot be obtained when a bimorph cell is used in the deflection device of a solid-state image sensor.